The present invention relates to moisture detectors, and particularly to an improved method and apparatus for non-contact measuring of moisture in moving material.
Moisture measuring devices typically employ a means for contacting the material being tested to establish a conductive connection, so that moisture in the material can be detected by electrical conduction. Unfortunately, the contacting means, typically brushes, are subject to damage, breakage and electrical shorting whereby the moisture indications tend to become erratic and inaccurate. Further, even if the brushes are in good condition, the degree of electrical contact provided with the material under test is often nonuniform.
Moisture detectors have been developed which do not require contact with the material being tested, but instead employ capacitive coupling or the like. Many such detectors, however, are quite sensitive to the position of the moving material relative to the sensor conductor, as well as to the thickness of the material, and therefore indications derived on a production-line basis can be somewhat undependable. Furthermore, the conveying means upon which the material is transported can short-circuit the measuring system such that a dependable reading cannot be obtained.
U.S. Pat. No. 4,377,783 discloses a measuring system for detecting moisture in wood veneer wherein transmitting and receiving plates are offset along the path traversed by the material being measured, and a conductive path to ground through the veneer and the conveyor moving the veneer is employed as part of the detection circuit. While efficacious, there is some dependency upon accurate contact with the material being measured.
U.S. Pat. No. 4,563,635 discloses a system for measuring moisture in wood veneer strips wherein the strips pass between arrays of plates including a transmitting plate on one side of the veneer and a receiving plate juxtaposed on the other. Phase plates on either side of and coplanar with the transmitting plate are energized by a signal having the reverse phase to that applied to the transmitting plate. When wet veneer passes between the plate arrays, part of the transmitted signal is shunted, reducing the signal received by the receiving plate. While this approach reduces sensitivity to vertical position of the veneer strips, eliminates problems associated with accidental grounding of the veneer, and obviates the need for mechanical contact with the veneer, in practice, some unwanted signal coupling occurs as a veneer sheet initially enters and then leaves the measuring region between the plate arrays, resulting in a false moisture detection signal.
U.S. patent application Ser. No. 638,020, now U.S. Pat. No. 4,683,418, assigned to the same assignee as the instant invention, provides one solution to the above-noted problem wherein transmitter bars connected to earth ground face the veneer opposite sensor plates phased with respect to signal ground; however, the bars ordinarily should be spaced relatively far apart making it somewhat difficult to detect moisture in small pieces of veneer.
It would therefore be desirable to provide a moisture measuring method and apparatus which are relatively insensitive to positioning of varying sized veneer along the path between the transmitting and receiving conductor arrays.
It is an object of the present invention to provide an improved method and apparatus for detecting moisture in wood veneer or the like.
It is a further object of the present invention to provide an improved method and apparatus for detecting moisture in an object in a manner less sensitive to physical position of the object.